The development and characterization of an atmospheric pressure, capacitively coupled plasma (CCP) torch for atomic absorption spectrometry (AAS) has been specifically described [D. C. Liang and M. W. Blades, Anal. Chem. 60, 27 (1988)]. Subsequent work has demonstrated that this device can also be used quite effectively as a source for atomic emission spectrometry (AES) [D. C. Liang and M. W. Blades, Abstracts, The Pittsburgh Conference & Exposition, paper No. 415 and 1140 (1988)]. The configuration described previously was designed for the analysis of small volumes of liquid samples of a size typically analyzed by electrothermal atomization AAS (5-50 .mu.L). However, the CCP can also be combined with other sample introduction techniques including laser ablation. The CCP developed for AAS was characterized by a long path length (20 cm) small diameter plasma sustained by capacitive coupling. The plasma could be operated at support-gas flow rates as low as 0.2 L/m and at radio frequency (rf) input powers between 30 to 600 W. Both the long path length tube geometry of the discharge and low support-gas flow rates acted to maximize analyte residence time in the plasma resulting in detection limits in the ng/L range.
By far the most important commercial spectral lamp for AAS and AFS is the hollow cathode lamp (HCL). The main advantages of the HCL are its very small spectral line-width and its high signal to background ratio. However, the absolute intensity of emission from the HCL is relatively low compared with the radiation from other plasma sources. To overcome this problem techniques such as direct current (dc) boosted-HCL, rf boosted-HCL, microwave coupled HCL, and high current pulsed HCL have been developed [Improved Hollow Cathode Lamps for Atomic Spectroscopy 1985, Ed. S. Caroli, Ellis Horwood Limited]. Additionally, the intensities of ion lines in HCLs are very weak, due to the dominant population of ground state atom in glow discharges [J. A. C. Broekaert, J. Anal. At. Spectrom. 2, 537 (1987)]. Traditional AAS primarily makes use of atomic resonance lines; however there is a large population of ground state analyte ions in ICP's even at relatively low powers [G. Gillson and G. Horlick, Spectrochim. Acta 41B, 431 (1986)]. The development of an intense ion line spectral source has some significance in this area in that it could assist in the reduction of source induced shot noise, consequently improving the detection limits for plasma source AAS.
There has recently been much interest in the application of sputtering sources in atomic spectrometry [P. Hannaford and A. Walsh, Spectrochim. Acta 43B, 1053 (1988)] [H. J. Kim and E. H. Piepmeier, Anal. Chem. 60, 2040 (1988)] [A. E. Bernhard, Spectroscopy, 2 No. 6, 24 (1987)]. Sputtering is the ejection of material from a surface caused by bombardment with an energetic beam of particles [B. Chapman, in Transactions of the Conference and School on the Elements, Techniques and applications of sputtering, 1 (1969)]. DC sputtering in a glow discharge source allows one to analyze solid samples by atomizing the analytes directly from the solid state. This approach offers some advantages. The time-consuming sample decomposition step can be omitted and analysis can be carried out without addition of reagents and without any separation and/or concentration steps so the risks of introducing contaminants and the loss of the element to be determined are considerably reduced. As a consequence an analysis can be carried out quite rapidly. It would appear that the sputtering rate should be a direct function of gas pressure, since the higher the pressure the more ions which would be available for sputtering. However, sputtering is usually carried out at pressures between 5.times.10.sup.-3 and 1 torr since glow discharges extinguish or switch over to arc discharges at higher pressures and the main sampling mechanism in arcs is thermal evaporation.
Although rf sputtering is not widely used as a sample introduction method in atomic spectroscopy, it has long been recognized as an important technique in sputter etching and chemical vapour deposition [B. Chapman, in Transactions of the Conference and School on the Elements, Techniques and applications of sputtering, 1 (1969)]. Rf sputtering at low pressures first suggested by Wehner in 1955 [G. K. Wehner, Advances in Electronics and Electron Phys., 7, 239 (1955)] and demonstrated in 1962 [G. S. Anderson, W. N. Mayer and G. K. Wehner, J. Appl. Phys., 33,2991 (1962)] has become a standard method for etching materials in the semiconductor industry. Atmospheric pressure rf sputtering was previously used by the inventors to supply Fe to the CCP discharge for the purpose of making temperature measurements. More recently, Stephens [R. Stephens, J. Anal. At. Spectrom. 3, 1137 (1988)] has described an rf discharge between two metal electrodes at atmospheric pressure, operating in helium at a power of 5-30 W. The sputtering effect of the discharge was deduced by observing atomic emission from the plasma and atomic absorption within the plasma. Stephens pointed out that this device offered a convenient means of observing either emission or absorption for those elements for which sputtering was not inhibited by the presence of a stable oxide layer.